Accidents, automobile and other types, have a significant impact on society's welfare. Two identified risk factors of the high accident rates are excessive daytime sleepiness (EDS) and ethanol. Many sleep disorders patients chronically suffer with EDS and their ability to function is significantly compromised. More importantly, many healthy people periodically experience daytime sleepiness due to insufficient and fragmented sleep which reduces their ability to function efficiently and increases their risk of injury and fatality resulting from ethanol related accidents is well documented. We now have evidence showing that level of sleepiness/alertness modulates the effects of ethanol on waking function. Increasing alertness reduces ethanol's risks, while increasing sleepiness exacerbates those risks. In all of the studies evaluating the interaction of ethanol and sleepiness/alertness, a residual sedation, that is sedation after ethanol is no longer detectable in breath or blood, was observed. Understanding residual sedation is important because it could pose a significant risk for accident and injury since it is believed that when breath ethanol concentration is zero one so no longer impaired. Studies are proposed to evaluate the dose relations and the duration of residual sedation after ethanol. These studies will define the nature and extent of the risks of ethanol related sedation. Preliminary data suggest that residual sedation is not a specific effect of ethanol or its metabolites, but is common to one other sedating drug and probably is the result of activation of mechanisms involved in sleepiness. Whether or not other drugs which have sedating effects will also show residual sedation will be systematically evaluated. Further, whether or not other causes of sedation will interact with residual sedation is to be assessed. These studies will determine the specificity of the ethanol related residual sedation. Finally, if residual sedation is a non-specific effect, sleep should reverse it and the characteristics of that sleep should be similar to that seen after sleep loss.